It is challenging to interconnect the large and rapidly growing number of servers in data centers with low cost. In current practice, tree is used and expensive high-speed switches are required in top levels of the tree. As an alternative solution, interconnection intelligence can be put on servers. Commodity servers in data centers often have at least two NIC ports, but only one is used for operation in current practice and the other is for backup. In this paper, we design FiConn, an interconnection structure for data centers which exploits the backup port on each server and eliminates the requirement of any switches other than the lowest-level commodity ones. Although with the server node degree of only two, FiConn is highly scalable to encompass hundreds of thousands of servers, while at the same time has low diameter and high bisection width. The basic routing algorithm in FiConn makes a balancing use of different levels of links. To further utilize the link capacities according to traffic states, we design traffic-aware routing in FiConn with little control overhead. The traffic-aware path is computed hop-by-hop by each intermediate server based on the available bandwidths of its two outgoing links. Simulation results demonstrate that the traffic aware routing indeed achieves high networking throughput.